This invention relates to the treatment of molten metals and, more particularly, to a method and apparatus for agitating molten metal and for incorporating alloying metal into a metal melt.
In the processing of molten metals prior to casting in moulds, molten metal is often held in a vessel such as a mixing furnace or holding vessel. The molten metal in such vessel is usually agitated which results in the formation of an amount of dross. The molten metal is cast as a base metal, or as a base metal alloy after the addition of one or more alloying metals to the melt of base metal. The dross formed in the treatment of molten metal and in the alloying of metals is a disadvantage in that dross must be retreated adding to the cost of the production of metals and alloys.
In the alloying of metals, one or more alloying metals are often added to a bath of molten base metal as elemental metal or master alloy in solid form such as in the form of particulates. In the production of most alloys, however, the alloying does not proceed readily. Such is particularly the case when a base metal is alloyed with alloying metals that have a relatively high melting point compared to that of the base metal. When such high melting alloying metals are added to molten base metal, the rate of dissolution is low and a high temperature increment over the melting temperature of the base metal or the alloy must be provided to increase the rate. In addition, in most cases, the base metal and added alloying metals form semi-plastic masses which are slow to dissolve and which accumulate in the bottom of the alloying vessel. Efforts to accelerate the dissolution by known methods such as vortex, high-velocity, or high-shear mixing are only partly successful, such mixing often also causing formation of considerable amounts of dross. Dross must not only be removed and treated, but also may contaminate the alloy product. Dross formation is aggravated by exposure to oxidizing conditions. As a result of these problems, the alloy products not only have a relatively large standard deviation from the desired composition specification but sometimes do not meet the specification and must be reprocessed. These problems tend to restrict the alloying to more costly batch processes.